Dissimilar metal welded joint with protective overlay



Dec. 6, 1960 F. T. EBERLE 2,963,129

DISSIMILAR METAL WELDED JOINT WITH PROTECTIVE OVERLAY Filed Dec. 9, 1954FERRITIC ALLOY STEEL AUSTENITIC ALLOY STEEL FIG. 2

FERRITIC ALLOY STEEL AUSTENITIC ALLOY STEEL FERRITIC ALLOY STEELAUSTENITIC ALLOY STEEL INVENTOR FRITZ T. EBERLE' ATTORNEY of the steamgenerators.

lot the "pile, those due DISSIMILAR METAL WELDED JOINT WITH PROTECTIVEOVERLAY Filed Dec. 9, 1954, Ser. No. 474,175 7 Claims. Cl. 189-36 Thisinvention relates to the production of a welded joint between austeniticand ferritic materials suitable for high temperatures, high "pressureservice under conditions involving thermal shock and cyclic temperatureand load applications.

Such conditions are encountered in high temperature process plants such.as, for example, oil refineries, in vapor or steam generators, and inheat exchangers of various types. The particular problems in any onetype of installation may differ in one or more aspects from those inanother type. Thus, refineries involve high temperatures but onlymoderate pressures, in conjunction with alternating oxidation andreducing conditions, corrosive environments and the like. In steamgenerators a more complex stress condition exists due to the combinedactions of high operating pressures and high operating temperatures,which are further aggravated by cyclic variations in these factors.While the invention is of general application under high temperature,high stress conditions in any type of installation, particular referencewill be made, by way of example only, to thehigh temperature and highstress conditions encountered in steam generators.

In order to obtain higher efficiencies, the outlet steam temperaturesand the operating pressures of central station steam generators havebeen constantly increasing, and presently some central station steamgenerating units have outlet temperatures of 1050 F. and operatingpressures of over 2000 p.s.i. The increasing useof such hightemperatures and pressures has brought with it problems .of providingmaterials and joints between such materials which will successfullywithstand the stresses encountered ithereat.

The long time load carrying characteristics of metals at hightemperatures, together with the economics in- "volved, have led steamgenerator designers to use both austenitic and ferritic materials forthe outlet components material may be used in the superheater and itssupports, and in the main steam line from the generator to the turbine.Use of both types of materials in the same component requiresthat'particular attention be given to the junctions between thesematerials, which junctions must operate under the particular temperatureand stress conditions encountered in producingsteam at relatively hightemperatures. In a superheater, for example, the external surface andthe superheater support lugs are at a higher temperature than'theinternal surface of the superheater tubes, due to the highertemperatures of the heat- ;ing gases as compared to the temperature ofthe steam flowing through the superheater. The reverse is true withrespect to the steam line leading to the turbine.

Operation under stress at such high temperatures intro- 'c luces manyproblems due to the difierential expansion and contraction of thedissimilar materials on either side joint, their relative surface andstructural stability, Aside from mechanical stresses, such as, for exam-For example, both types of Patented Dec, 6, 1960 traction, the factorsinfluencing the service life of welded basically of a metallurgicalnature, such as carbon depletion in the heat affected zone of theferritic material, notching due to oxide penetration occurring therein,micro-fissuring in the weld junction, and accelerated creep due to theseconditions. Examples of joints between ferritic and austeniticmaterials, with which these problems are encountered, are the joining ofa ferritic alloy having substantially 2%% chromium to an austeniticalloy of the 18-8 or 25-20 type.

The present invention is particularly directed to the production of awelded joint, between austenitic and ferritic materials, which has therequisite strength, ductility, and oxidation resistance to assuresatisfactory performance under conditions of high stresses occasioned bythermal shock, cyclic temperature and load application, and differentialthermal expansion. It has been found that these objectives can beobtained by Weld uniting a low alloy ferritic member to an austeniticmember by a weld deposit which is carbide-stabilized, has an oxidationresistance which is higher than that of the ferritic member, and acoeflicient of thermal expansion intermediate the coefficients ofthermal expansion of the two members, the weld deposit including anoverlying heat resistant oxygen barrier. The welded joint is a compositedeposit of two or more metals, at least one of which is acarbidestabilized and oxidation-resistant ferritic metal and at leastone more of which is an austenitic, heat-and-oxidation-resistant metal.

In one embodiment of the invention, the weld groove forming surface ofthe austenitic member is buttered with a carbide-stabilized, ferriticmetal having an oxidation resistance higher than that of the ferriticmember. The remainder of the weld groove is then filled with a ferriticmetal of substantially the same composition as that of the ferriticmember, and which may be carbidestabilized. At least theferritic-austenitic junction of the welded joint is then covered with aweld deposit overlay of an austenitic heat-resisting metal having a heatexpansion coefiicient which is intermediate the heat expansioncoeflicients of the two base members.

In another embodiment, the weld groove between the ferritic andaustenitic base members is filled with a carbide-stabilized, ferriticmetal having a higher oxidation resistance than that of the ferriticbase member. At least the ferritic-austenitic junction of the Weldedpoint is then provided with substantially the same overlay as in thefirst embodiment. L

In a further embodiment, the weld groove forming surface of the ferriticmember is buttered with a carbide-stabilized, ferritic metal having anoxidation 'resistance higher than that of the ferritic member. Theremainder of the weld groove is then filled with anaustenitic,heat-resisting metal having a coeflicient of thermal expansionintermediate those of the base members, the weld depositio'n of thislatter metal being continued to form an overlay thereof across theferritic austenitic junction.

For a more complete understanding of the invention principles, referenceis made to the following description of a typical embodiment thereof asillustrated in the accompanying drawings.

In the drawings:

Figs. 1, 2 and 3 are partial transverse sectional views of weldedpoints, each weld uniting a low alloy. ferritic base member to anaustenitic base member, and embodying the invention. a

' -In the following description of the invention, the term 1ferriticalloy' is used to designate alloys having compositions such ascarbon--0.5% molybdenum, 2% Crto differential thermal expansion and con-=3 similar predominantly ferritic alloy steels. Similarly, the termaustenitic alloy refers to alloys known to those skilled in the art as18-8 (18 Cr-8 Ni), 18-8 Cb, 18-8 Ti, 25-12, "25-20, or any otheralloy-steel which is predominantly austenitic in structure.

Referring to Fig. 1 of the drawing, a predominantly ferritic alloy steelworkpiece 10, such as a pressure tube or pipe, is illustrated as weldunited to a predominantly austenitic alloy steel workpiece 20, such as apressure tube or pipe, by a fusion deposited, carbide-stabilized, weldedjoint 30'having an oxidation resistance which is higher than that ofmember 10 and a coefiicient of thermal expansion intermediate thecorresponding coefficients of members 10 and 20, joint 30 including anoverlying heat-resistant oxidation barrier 35. By way of example only,ferritic member 10 may be an alloy steel containing 2%% chromium and 1%molybdenum, this .steel being known commercially as fCroloy 2%, andaustenitic member 20 may be an 188Ti alloy steel. The facing endsurfaces of members 10 and 20 are suitably bevelled, as at 11 and 21,respectively, to form a weld groove, a backing member 15 being arrangedto bridge the open lower end of the welding groove.

In this embodiment of the invention, the 'weld groove forming surface 21of austenitic alloy steel member 20 is first buttered with a fusiondeposit 31 of carbide- .stabilized ferritic metal having an oxidationresistance higher than that of ferritic alloy steel member 10. Thisdeposit or layer 31 resists carbon migration, due to its beingcarbide-stabilized, and also resists oxidation at the relatively highand cyclically varying working temperatures to which joint 30 is exposedin practice. If, by way of example, member 10 is a low alloy steelcontaining 2%% Cr and 1% M or 3% Cr and 1% M0, the metal deposited toform buttering layer 31 may be a 5% Cr0.5% Mo alloy steel stabilizedwith Cb (a S Cr.5 .MoCb steel).

The remainder of the weld groove between surface 11 of member and layer31 is then filled with a fusion deposit 32 of ferritic alloy steelhaving about the composition of member 10. In the specific examplegiven, deposit 32 may be a 2%.% Cr1% Mo ferritic alloy steel. Deposit 32may also be of carbide-stabilized ferritic alloy steel such as 254% Cr1%Mo-Cb alloy steel, for example.

As the final step in forming the welded joint or weld deposit 30, theprotective overlay 35 is fusion deposited at least across theferritic-austenitic junction, which latter .is the junction of butteredlayer 31 and surface 21 of member 20. This weld overlay is deposited ofan austenitic heat resisting steel alloy having a coefficient of thermalexpansion intermediate the corresponding coefficients of members 10 and20.

A suitable austenitic steel alloy for overlay 35 is a 25 -Cr-20 Nisteel, for example, or a 77 Ni15 Cr steel .known as Inconel. Anothersuitable austenitic steel'is an 80 Ni Cr steel. In the specific weldedjoint given by way of example, the cotficients of thermal expansion,from room temperature to 1200 F., are 7.69 X 10 for the Croloy 2% member10, and 10.41 x 10 for the :18-8Ti member 20. The correspondingcoefiicient for a Cr-20 Ni overlay is 9.20 x 10*, which is inter--mediate the coefiicients of the base members '10 and 20.

The heat resistant overlay 35 protects the relatively .vulnerableferritic-austenitic junction (21-31) from the extreme effects ofcyclically varying elevated temperatures during service. As theexpansion of the overlay, -with change in temperature, is intermediatethe expan- -sions of members 10 and 20, the tendency of the joint 'tofail due to differential thermal expansion of the base members issubstantially eliminated. Welded joints between dissimilar metalmembers, including overlay 35, -have a life as much as five times thelife of corresponding -joints without the overlay, when subjected tocyclically -.varying elevated, temperatures, 1

In the embodiment of Fig. 2, the entire welding groove is filled with afusion deposit 33 of carbide stabilized ferritic metal having anoxidation resistance higher than that of ferritic alloy steel member 10.By way of ex- 5 ample, deposit 33 may be the 5 Cr--.5 Mo-Cb ferriticsteel useful for the butten'ng layer 31 of Fig. 1. Deposit 33 resistscarbon migration, due to its carbide stabilization by the Cb, andresists oxidation at the filatively high and cyclical-1y varyingtemperatures to which the 10 joint is subjected in service.

At least the ferritic-austenitic junction 2133) is then covered with thefusion deposited overlay 35 in the same manner as in the embodiment ofFig. l. Overlay 35 may be one of the same heat resistant austeniticalloys as 15 used in forming the overlay 35 of Fig. 1.

In the embodiment of Fig. 3, the weld groove forming surface 11 offerritic member 10 is first buttered with a fusion deposit 34 ,of acarbide-stabilized ferritic steel alloy, such as the 5 Cr-.5 Mo-Cb steelpreviously 20 mentioned. The remainder of the weld groove is then filledwith a fusion deposit of an austenitic, heat resisting, steel alloyhaving a coeflicient of thermal expansion intermediate those of members10 and 20. For example, this may be the 25 Cr 20 Ni austenitic steelalloy, or

25 the Inconel" alloy, previously mentioned. The deposit 36 is continuedto form the overlay deposit 35 so that the austenitic-ferritic junction3436 is covered by such heat resistant austenitic steel alloy.

i The carbide-stabilized ferriticalloy steel deposit having higheroxidation resistance than that of the ferritic alloy steel member 10suchas deposits 31, 33 and 34- may, in general, have a higher chromium orsilicon content than those of member 10, and may also have a slightlylower coefficient of thermal expansion. While this is less favorablethan would be the case witha coefficient of thermal expansion somewhathigher than that of member 10, the slight decrease is insignificant andis out-balanced by the higher oxidation resistance. The combination ofthe carbide-stabilized and higher oxida- 40 tion resistance deposit withthe oxygen barrier over the dissimilar junction provides an effectivesafeguard against stress-oxidation or stress-oxidation-fatigue cracking.

While specific embodiments of the inventionhave been shown and describedin detail to illustrate the application 5 of the invention principles,it should be understood that the invention may be otherwise embodiedwithout departing from such principles.

What is claimed is: a

1. A composite welded assembly having an extended service life whensubjected to cyclically varying elevated temperatures of the order of1100 F. and higher, said assembly comprising, in combination, a ferriticalloy steel member having a weldv groove forming surface; an austeniticalloy steel member having a weld groove forming surface; and a fusionwelded joint uniting said members comprising afusionweld deposit fusedto .bothof said surfaces and including afusion weld section of a carbidestabilized ferritic alloy steel having an oxidation resistance .higherthan that of said ferritic steel member, and a fusion weld section of aheat and oxidation resistant austenitic alloy steeljsaid ferriticalloysteel section of said deposit beingfu sedtoatleast one of saidsurfaces and said austenitic alloy steel section of said deposit beingfused to said ferritic alloy steelsection of-saiddepositand overlyingandoverlapping theferritic-austenitic junction of said assembly.

2. A composite welded assembly having an extended service life whensubjected to cyclically-varying elevated temperatures of the order ofl100 F. andhigher, said -assembly comprising, in combination, a ferriticalloy steel member. having a weld'groove forming surface;.,an austeniticalloy steel-memberhavinga weld groove forming surface; and a fusionweldedjointunitingsaiimemhers comprising a fusion weld deposit. fused toboth of said Surfaces and including a fusion weld section bfj-a carbidestabilized ferritic alloy steel having an oxidation resistance higherthan that of said ferritic alloy steel member, and a fusion weld sectionof a heat and oxidation resistant austenitic alloy steel having acoeflicient of thermal expansion intermediate those of said members;said ferritic alloy steel section of said deposit being fused to atleast one of said surfaces and said austenitic alloy steel section ofsaid deposit being fused to said ferritic alloy steel section of saiddeposit and overlying and overlapping the ferritic-austenitic junctionof said assembly.

3. A composite welded assembly having an extended service life whensubjected to cyclically varying elevated temperatures of the order of1100 F. and higher, said assembly comprising, in combination, a ferriticalloy steel member having a weld groove forming surface; an austeniticalloy steel member having a weld groove forming surface; and a fusionwelded joint uniting said members comprising a fusion weld deposit fusedto both of said surfaces and including a fusion weld section of acarbide stabilized ferritic alloy steel having an oxidation resistancehigher than that of said ferritic alloy steel member, and

' a fusion weld section of a heat and oxidation resistant austeniticalloy steel having a coefficient of thermal expansion itnermediate thoseof said members; said ferritic alloy steel section of said deposit beingfused to said austenitic alloy steel member surface and said austeniticalloy steel section of said deposit being also fused to said austeniticalloy steel member surface and overlying and overlapping theferritic-austenitic junction of said asscmbly.

4. A composite welded assembly as claimed in claim 1 in which saidferritic alloy steel section of said deposit is deposited against thegroove forming surface of said austenitic member; and including a fusiondeposit of a ferritic alloy steel, of substantially the same compositionas the ferritic alloy member, deposited between said first deposit andthe groove forming surface of said ferritic member.

5. A composite welded assembly as claimed in claim 1 in which saidferritic alloy steel section of said deposit is deposited against thegroove forming surfaces of both of said members. 1

6. A composite welded assembly as claimed in claim 2 in which the fusionweld deposit further includes a fusion weld section of a ferritic alloysteel having substantially the same composition as said ferritic member.

7. A composite welded assembly as claimed in claim 3 in which said welddeposit further includes a fusion weld section of a ferritic alloy steelhaving substantially the same composition as said ferritic member.

References Cited in the file of this patent UNITED STATES PATENTS1,808,205 Bryant et al. June 2, 1931 1,959,791 Kautz June 22, 19342,054,939 Larson Sept. 22, 1936 2,158,799 Larson May 16, 1939 2,209,290Watts Jan 23, 1940 2,240,672 Scherer et a1 May 6, 1941 2,759,249 EberleAug. 21, 1956 2,769,227 Sykes et al. Nov. 6, 1956

